<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        h3.title{
            color: red;
        }
        div.box{
            width: 100px;
            height: 100px;
            background-color: red;
        }
        div.box.box1{
            border: 3px solid green;
        }

    </style>
</head>
<body>
    <h3>我是h3标签</h3>
    <h3 class="title">我是h3.titel标签</h3>
    <div class="box box1"></div>
    <div>我是div</div>
    <div class="box"></div>

</body>
</html>